For various applications it is useful to measure true ground velocity in forward and sideward directions, vehicle front to rear level, and height of a vehicle. Furthermore, it is also useful to measure the ground cover or surface conditions to predict if there is snow, rain or other coatings on the road surface. These metrics can be used in vehicle navigation, antiskid braking, and ride leveling systems. A class of prior art approaches use radar apparatus that overcome many disadvantages of other measurement methods. However, contemporary radar based vehicular ground speed sensors have their share of problems primarily related to complexity and lack of full function.
For one, without significant complexity prior art approaches measure ground velocity in one dimension only--such as forwards-rearwards. Furthermore, the accuracy of these systems is relatively poor.
One arrangement of a radar based vehicle-ground surface sensor is a monostatic configuration. In this type of configuration a singular antenna transmits a signal from underneath a vehicle to a ground surface. A portion of the transmitted signal is reflected back from the ground surface to the antenna and is analyzed. A frequency of the received signal will differ from the transmitted signal dependent on a speed of displacement between the vehicle and the ground surface. The speed vs. frequency phenomena is known as the Doppler effect, and the measurement of the reflected signal is known as backscatter measurement. Although this approach is relatively simple, the nonostatic configuration is not capable of distinguishing a change in the vehicle level and/or vehicle height perpendicular to the ground surface, from a change in speed planar with the ground surface. This deficiency is particularly troublesome when measuring true ground speed on rough roads because the vehicle height will change frequently as the vehicle bounces. Additionally, dynamic vehicle tilt, or pitch attitude may be interpreted as a change in planar speed. At the same time there is no mechanism for measuring vehicle height in this type of system independent of planar speed measurement. Furthermore, the received signal strength in a backscatter measurement arrangement can be very low with some ground surfaces, requiring a significant investment in complexity of the required signal processing circuitry. Although a mounting angle of the antennas can be changed to increase backscatter it is done so at the expense of adding perpendicular speed error.
One approach to solve the perpendicular speed errors, associated with dynamic changes in vehicle height, has been to use plural antennas. Plural antenna arrangements include an approach that transmits and receives signals downwardly to the ground surface in opposite directions, usually in forward and rearward directions at equal and opposite angles displaced from a vertical plane. This arrangement is a so-called "Janus" configuration. A purpose of the "Janus" configuration is to eliminate the effect of vertical measurement error caused by a change in the difference between the vehicle height from the ground surface, by adding and subtracting the received backscatter signals to derive sum and difference signals from which planar speed is determined. The "Janus" type of approach is complex and costly because of the two complete antenna and transceiver systems. Furthermore, this arrangement only measures velocity in one plane of travel--such as forwards-backwards, and has no means to measure height of a vehicle or ground surface conditions.
Another plural antenna approach uses a separate transmit and receive antenna. This arrangement is known as a bi-static configuration. Known prior art systems configured the separate transmit and receive antenna in a backscatter measurement arrangement. A disadvantage of this approach is that the angle to the surface changes with vehicle ride height, which causes speed inaccuracies when computing a Doppler return effect. As with the Janus configuration this prior art arrangement only measures velocity in one plane of travel--such as forwards-backwards, and has no means to measure height of a vehicle or ground surface conditions.
What is needed is an improved approach for measuring various vehicle-ground surface parameters such as vehicle planar velocity in orthogonal directions, true ground speed, vehicle front to rear tilt, vehicle-ground surface height and various road surface conditions such as rain and snow.